1. Field of the Invention
This invention relates to a disconnected piston for use in valve actuators that are used to operate valves, especially gate valves, that are operated between a first position in which fluid flows through the valve bore and a second position in which the valve bore is closed. Such valves have a variety of applications in the oil and gas industry such as controlling the flow of hydrocarbon fluids through well heads, controlling fluid flow in pipelines and a most critical operation, controlling the flow of pressurized control fluid to and from pressure actuated assemblies such as blowout preventers or other valve actuators.
When these valve actuators are used in a subsea environment, their proper operation is particularly critical as the valves being actuated are positioned on assemblies called subsea trees at great water depths where human intervention is extremely difficult. As the search for oil and gas in offshore locations has grown, it has been directed into deeper and deeper water depths. Today oil and gas exploration and production operations in water depths of 6,000 to 10,000 feet is becoming routine, with deeper water systems planned.
At these extreme water depths, the ambient pressure of the sea water becomes a factor in the design and operation of the aforementioned valve actuators. At these depths the ambient pressure of the water can interfere with the operation of the typical subsea valve actuator. These valve and actuator assemblies are typically lowered from the surface with their interior pressure chambers, acting through the control system, experiencing the full hydrostatic head of the water, which is approximately 3,000 psi at a 6,000 foot water depth. These high pressures can cause the valve actuator to malfunction or fail when the equipment is lowered to its operating depth as a result of excessive reverse pressure acting on the piston of the actuator.
2. Description of Related Art
U.S. Pat. No. 4,135,547 to N. H. Akkerman et al. shows a gate valve actuator with a secondary annular chamber that acts as an accumulator and a secondary back-up system for operating the valve.
A dual stage valve actuator for operating a gate is disclosed in U. S. Pat. No. 4,934,652 to T. A. Golden. This apparatus uses a two-part piston to provide a high initial force for overcoming frictional forces.
U.S. Pat. No. 6,125,874 to D. G. Holliday shows a gate valve actuator using valve body pressure with a floating ring to provide additional force for moving the valve to its fail-safe position.
A gate valve actuator design using a dual piston design to control the closing force of the valve actuator is shown on the Internet at www.rotator.no/products/actuator.html.
The present invention comprises a gate valve actuator utilizing a disconnected piston design that separates under conditions of excessive actuator internal pressure to prevent overloading and damaging the threaded connection between the valve stem and actuator piston. The hydraulically actuated valve actuator comprises a spring housing including a bonnet for coupling the spring housing to a valve. The bonnet includes a valve stem bore extending there through and a seal assembly disposed around the periphery of the valve stem bore for sealing a valve stem extending through the bonnet. The spring housing includes an upper head and a packing gland that are sealingly attached to the spring housing to form an interior pressure chamber.
The interior pressure chamber has a pressure responsive actuator piston disposed therein with a large coil spring between the spring retainer nut and the packing gland. The spring retainer nut is connected to an adapter stem which is then threaded onto a valve stem extending through the bonnet to a valve gate. The coil spring is designed to move the valve gate from a normally open position to a fail closed position. The pressure responsive actuator piston includes a piston and a spring retainer nut that are separable after closing of the valve to prevent ambient pressure acting within the pressure chamber on the pressure responsive actuator piston from overloading the valve stem to piston coupling. The packing gland includes a valve stem bore therethrough and a seal ring disposed around the valve stem bore for sealing the valve stem extending through the packing gland. The actuator is attached to a gate valve body by a plurality of tie bolts. The gate valve is of a conventional construction with a fluid flow bore therethrough and a gate member movable between first and second positions for opening and closing the fluid flow bore. The valve stem is coupled to the gate member by suitable means as a T connection or buttonhead connection.
A principal object of the present invention is to provide a valve actuator for use in environments where the load generated by the pressure acting in reverse on the pressure responsive piston of the valve actuator is not transmitted to the valve stem and thereby ensuring the valve stem is not overloaded.
Another object of the present invention is to provide a valve actuator that is suitable for use with a subsea control system that ensures the hydrostatic pressure load generated by control system does not overload or damage the valve actuator to stem connection.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.